94 CHARLES R. STOCKARD 



circulation never hatch although they may often live for more 

 than thirty days. 



All recent investigators have claimed that there are no blood 

 islands present on the Teleostean yolk-sac. Yet the presence 

 of such islands is readily demonstrated in living Fundulus 

 embryos, in normal specimens as well as in those with no cir- 

 culation. 



2. The plasma or fluid in the embryos failing to develop a 

 circulation begins to collect at an early time in the body cavities. 

 The pericardium becomes hugely distended with fluid as well as 

 the lateral coelomic spaces and the Kupffer's vesicle at the pos- 

 terior end of the embryo. The great distension of the pericardium 

 due to this fluid accumulation pushes the head end of the embryo 

 unusually far away from the surface of the yolk. The heart 

 is thus stretched into a long straight tube or string leading 

 from the ventral surface of the head through the great pericardial 

 cavity to the anterior yolk surface (compare figures 15 to 20). 



No blood vessels form on the extreme anterior portion of the 

 yolk-sac, so that the venous end of the heart is never connected 

 with veins, and does not draw fluid into its cavity to be pumped 

 away through the aorta. When the heart cavity does contain 

 fluid it is unable to escape and small floating particles may often 

 be observed rising and falling with the feeble pulsations of the 

 heart. 



3. The hearts in embryos without a circulation are lined by 

 a definite endocardium, but the myocardium is poorly developed, 

 sometimes consisting of only a single cell layer. Chromatophores 

 are not present in the wall of the normal heart but in the experi- 

 mental hearts these large cells ladened with pigment granules 

 are invariably found. The cavity in many of the hearts is 

 almost if not entirely obliterated by the presence of periblastic 

 material and large amorphous periblast nuclei. 



The conus end of such hearts leads directly to a more or less 

 closed ventral aorta, portions of the aortic arches are seen in 

 the sections as open spaces, and dorsal aortae are almost invari- 

 bly seen as typical spaces lined by characteristic embryonic 

 endothelium. 



